/* 已知一致宽度 */
.wrapper {
  position: relative;
}
.wrapper .box {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 100px;
  height: 100px;
  margin: -50px 0 0 -50px;
}
/* 未知宽度 */
.wrapper {
  position: relative;
}
.wrapper .box {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}
/* flex布局 */
.wrapper .box {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100px;
}
